The U.S. Supreme Court this week will consider whether a landmark civil rights law, the Voting Rights Act, remains constitutionally valid, given the growth in the political power of minority voters and candidates.

Civil rights groups fear the court's conservatives are prepared to gut what the ACLU calls "the most important piece of civil rights legislation Congress has ever enacted."

The justices will hear oral arguments in the case Wednesday and rule sometime before the current court term ends in late June.

Passed by Congress in 1965 and renewed four times since then, most recently in 2006, a key provision of the law requires states with a history of discrimination at the polls to get federal permission before making any changes to their election procedures — from congressional redistricting to changing the locations of polling places.

The law was at the core of last year's successful efforts to block strict voter photo ID laws in Texas and South Carolina and to prevent Texas from redrawing its legislative and congressional boundaries in a manner that challengers claimed would have discriminated against minority voters.

"The last election vividly showed that voter suppression and voting discrimination are not just problems of the past. They continue to undermine our democratic process," says the ACLU's Steve Shapiro.

The challenge to the law comes from Shelby County, Alabama, a mostly white suburb south of Birmingham. It argues that the pre-clearance requirement — which covers nine entire states and 66 counties or townships in seven others — is unconstitutional.

The areas covered by the law, it says, include some localities that have made substantial reforms but leave out other parts of the country that have failed to root out discrimination at the polls.

"Florida has been forced into pre-clearance litigation to prove that reducing early voting from 14 days to 8 is not discriminatory, when states such as Connecticut, Rhode Island, and Pennsylvania have no early voting at all," says Bert Rein of Washington, DC, the lawyer for the county.

While the history of blatant discrimination at the polls justified renewing the law in the past, Shelby County says, Congress failed to marshal enough evidence in 2006 to justify extending it for another 25 years. "At most, the 2006 legislative record shows scattered and limited interference with voting rights, a level plainly insufficient" to sustain the pre-clearance requirement, Rein says.

Since 1990, adds Alabama’s Attorney General, Luther Strange, African Americans in the state have registered and voted in larger percentages than in states outside the South.

But the NAACP Legal Defense and Education Fund says the current map is a close enough fit to cover the areas of greatest concern. "Congress is not a surgeon with a scalpel when it acts to legislate across the fivty states, but it can reasonably attack discrimination where it finds it," the group says.

If the law were struck down, civil rights groups fear the areas covered by the law would revert to their old habits.

Warns the Leadership Conference on Civil and Human rights, “There is a significant risk of backsliding and a likelihood that millions of minority voters will face new barriers to the exercise of their most fundamental human right.”

President Obama expressed a similar sentiment in a radio interview last week. If covered jurisdictions no longer had to defend their electoral changes in advance, Obama said, civil rights groups would be forced to file lawsuits after voting changes were already in place.

“There are some parts of the country where obviously folks have been trying to make it harder for people to vote. So generally speaking, you’d see less protection before an election with respect to voting rights,” Mr. Obama said.

The Justice Department, which is defending the law before the Supreme Court, argues that the coverage formula is flexible, allowing local governments to bail out of the pre-clearance requirement if they can demonstrate they have not discriminated against minority voters for at least ten years.

During the past three decades, 38 bailouts have been granted, freeing 196 local jurisdictions of the preclearance requirement, the Justice Department says. They include the first ever granted from parts of Alabama, Georgia, Texas, and Virginia, four of the states that are otherwise covered by the law.

Four years ago, the Supreme Court strongly suggested that several justices had doubts about its constitutionality, given recent electoral reforms. "Things have changed in the South," the court said in 2009. "Blatantly discriminatory evasions of federal decrees are rare."

The court then went on to reject a constitutional challenge to the pre-clearance requirement, but it strongly suggested Congress should update the coverage formula. Because, however, no changes have since made, the court may prepared to go the rest of the way this time.